Flow control of exhaust gas through an ICE has been used in order to provide vehicle engine braking. Engine brakes may include exhaust brakes, compression release type brakes, and/or any combination of the two. The general principle underlying such brakes is the utilization of gas compression generated by the reciprocating pistons of an engine to retard the motion of the pistons and thereby help to brake the vehicle to which the engine is connected.
Exhaust brakes are known to be useful to help brake a vehicle, particularly heavy vehicles such as trucks and buses. Exhaust brakes may generate increased exhaust gas back pressure in an exhaust system, including an exhaust manifold, by placing a restriction in the exhaust system downstream of the exhaust manifold. Such restriction may take the form of a turbocharger, an open and closeable butterfly valve, or any other means of partially or fully blocking the exhaust system.
By increasing the pressure of the exhaust manifold, an exhaust brake also increases the residual cylinder pressure in the engine cylinders at the end of the exhaust stroke. Increased pressure in the cylinders, in turn, increases the resistance encountered by the pistons on their subsequent up-strokes. Increased resistance for the pistons results in braking the vehicle drive train which may be connected to the pistons through a crank shaft.
Exhaust brakes have been provided such that the restriction in the exhaust system is either fully in place or fully out of place due to the associated expense and complexity of a system with a variable restriction. These exhaust brakes produce levels of braking which are proportional to the speed of the engine at the time of exhaust braking. The faster the engine speed, the greater the pressure and temperature of the gas in the exhaust manifold and cylinders. The higher pressure and temperature result in increased resistance to the up-stroke of the piston in the cylinder and therefore, increased braking.
Since the exhaust system and engine cannot withstand unlimited temperature and pressure levels, the exhaust brake restrictions have had to be designed such that the operation thereof at a rated maximum engine speed will not produce unacceptably high pressures and temperatures in the exhaust system and/or engine. The restrictions have been designed such that they produce less than maximum temperatures and pressures, and less than maximum braking at engine speeds below the rated maximum speed. Accordingly, there is a need for a system and method for realizing increased exhaust braking at less than maximum engine speed using an exhaust restriction having a fixed size designed to produce maximum exhaust braking at the rated maximum engine speed.
Compression release brakes, or retarders, may be used in conjunction with, or independently of, exhaust brakes. Compression release retarders convert, at least temporarily, the cylinder of an internal combustion engine (of the compression ignition type for example) into an air compressor. A retarder converts an engine's kinetic energy into thermal energy by opposing the motion of the engine's pistons with compression developed in the cylinders. A compression release event may be initiated by a piston traveling through its up-stroke and compressing gas in the cylinder which opposes the upward motion of the piston. When the piston nears the top of its up-stroke, an exhaust valve can be opened to "release" the compression, thereby preventing the piston from recapturing the energy stored in the compressive heat generating up-stroke on the rebound of a subsequent expansive kinetic energy generating down-stroke. In this manner the kinetic energy of the piston is converted to thermal energy and conveyed from the engine through the exhaust system, resulting in a reduction of the engine's kinetic energy and an associated braking of the engine.
By repeating the compression release event in the engine's cylinders with each cycle of the engine, the engine develops retarding horsepower which helps brake the vehicle. This can provide a vehicle operator with increased control over a vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted compression release retarder can develop a retarding horsepower that is a substantial portion of the operating horsepower developed by the engine on positive power.
An example of a prior art compression release engine retarder is provided by the disclosure of the Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is incorporated herein by reference. Engine retarders, such as the Cummins retarder, employ after-market hydraulic systems to control the operation of exhaust valves to carry out the compression release event. These hydraulic systems may be driven and powered by the engine's existing valve actuation system, e.g., the rotating cams of an engine with a camshaft. When the engine is producing positive power, the hydraulic system is disengaged from the valve control system so that no release events occur. When compression release retarding is desired, the hydraulic system engages the exhaust valves to provide the compression release events.
Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) for Method and a Device for Engine Braking a Four Stroke Internal Combustion Engine, assigned to Volvo AB, and incorporated herein by reference, discloses a system for increasing the braking power of a compression release retarder by opening an exhaust valve before a compression release event to allow additional exhaust gas to flow into the cylinder, i.e., an exhaust gas recirculation system. In the Gobert system, the exhaust valve is limited to being opened a predetermined fixed amount to recirculate exhaust gas into the cylinder. Gober employs a fixed lash system. The Gobert system, therefore, is the same as the prior art exhaust brakes, in that the opening, closing and lift of the exhaust valve for recirculation must be fixed such that the temperatures and pressures attained when the engine is operating at a maximum speed do not exceed the thermal and pressure load limits of the engine. It follows that the temperatures and pressures (and therefore braking) will be less than would be potentially possible at a less than maximum engine speed.
The prior art also discloses systems for varying the amount of lash between a slave piston and an exhaust valve to be opened by the slave piston. For example, Applicant is aware of the following prior art lash systems which may be used to vary lash and to thereby advance the time of valve opening: Meistrick, U.S. Pat. No. 4,706,625 (Nov. 17, 1987) for Engine Retarder With Reset Auto-Lash Mechanism; Hu, U.S. Pat. No. 5,161,501 (Nov. 10, 1992) for Self-Clipping Slave Piston; Custer, U.S. Pat. No. 5,186,141 (Feb. 16, 1993) for Engine Brake Timing Control Mechanism; and Hu, U.S. Pat. No. 5,201,290 (Apr. 13, 1993) for Compression Release Engine Retarder Clip Valve, all of which are incorporated herein by reference. While valve lash adjustment systems for advancing the time of valve opening exist, such systems are limited to (I) making the valve open earlier, close later and increasing lift, or (ii) making the valve open later, close earlier and decreasing lift. The lash systems do not enable independent control of the time a valve is opened and closed, which may be necessary to obtain optimal exhaust gas recirculation for temperature and pressure control in the engine compatible with optimal braking at various engine speeds.
None of the prior art methods and systems teach or suggest that the opening and closing of an exhaust valve may be controlled independent of each other to optimize exhaust gas recirculation for engine braking at various speeds. Furthermore, control of exhaust gas recirculation by selective variable levels of back pressure (i.e., Exhaust Pressure Regulation (EPR)) is also not taught. If the amount of exhaust gas recirculation were controlled (which it is not in Gobert) through independent control of exhaust valve opening and closing, the levels of pressure and temperature in the exhaust manifold and engine cylinders may be maintained such that optimal degrees of engine braking are attained at any engine speed. Since vehicles typically are required to undergo braking at any and all engine speeds, there is a need for a system and method of controlling the amount of exhaust gas recirculated to an engine cylinder.
The prior art methods or systems also do not teach or suggest that the opening and closing of an exhaust valve for exhaust gas recirculation may be controlled in response to the levels of various engine parameters, such as temperature, pressure and engine speed, so that the levels of such parameters may be regulated. There is accordingly a need to control exhaust gas recirculation in accordance with one or more engine parameters, such as temperature, pressure, and engine speed, etc., so that levels of engine braking which "push the limit" of such parameters may be attained for any engine speed. By monitoring such parameters and controlling the exhaust gas recirculation in response to the monitored levels of such parameters, the maximum allowable pressures and temperatures (and therefore maximum braking) may be reached for any engine speed.
Other exhaust gas recirculation systems and methods have not recognized the impact of varying the overlap between the time an exhaust valve is opened for recirculation and the time an intake valve is opened for intake. The exhaust valve may be opened for exhaust gas recirculation during the time the intake valve is opened on a downward intake stroke of a piston. The intake valve thereby provides an outlet during braking for high pressure gas flowing back from the exhaust manifold and into the cylinder. By varying the overlap of the opening of the intake and exhaust valves, the pressure and temperature of the exhaust manifold and cylinder may be controlled as well as the NOx emission of the engine.
Variation of the overlap of the intake and exhaust valve openings may also be controlled to regulate the level of noise produced by engine braking. Decreasing the overlap decreases the flow of gas and duration of the flow back through the intake valve and may accordingly decrease the level of noise emitted from the intake system of the engine.
It is apparent from the disclosures of the prior art that there remains a significant need for a method of controlling the opening and closing of an exhaust valve for exhaust gas recirculation in order to increase the effectiveness of and optimize compression release retarding and exhaust braking. Further, there also remains a significant need for a system that is able to perform that function over a wide range of engine operating parameters and conditions. In particular, these remains a need to "tune" compression release and exhaust brake systems to optimize their performance at operating speeds lower than the maximum rated speed of the engine in which they are used.